The present invention relates to an electronic device intended to be used in combination with the devices which make it possible to trigger the ignition in the cylinders of an internal combustion engine.
It is known that for satisfactory running of an internal combustion engine it is necessary to use a distributor, the rotation shaft of which is driven by the engine, this distributor producing an ignition signal which triggers the discharge of an ignition coil onto the particular spark plug which should be fed with a high-voltage current. The ignition signal is timed to have a shift relative to the top dead centre, TDC, position of the piston which is on its compression stroke and above which ignition must be caused. The shift of the ignition signal is a function of the rotation speed in accordance with a defined curve, and for the production of the ignition signal the distributor must include a sensor having a moving part driven by the rotation shaft of the distributor and subjected to angular shift making it possible to achieve the desired timing ignition delay.
Furthermore, it is known that under certain operating conditions an internal combustion engine experiences a knocking phenomenon which imposes highly undesirable vibrations on the pistons. If attempts are made to improve the efficiency of the engine by causing it to run systematically at the maximum of the torque curve, there is a risk that under certain running conditions the knocking phenomenon will occur. It has therefore been envisaged to identify the occurrence of the knocking phenomenon, in order to reduce, at this moment alone, the value of the ignition timing advance, thus causing the engine to run at maximum torque only for as long as the knocking phenomenon is not occurring. This requires a vibration sensor located on the engine/gearbox unit to identify the occurrence of knocking, (which occurs at frequencies of about 5 KHz). This sensor is associated with a bandpass filter in order to eliminate background noise but, to avoid any inopportune action of the sensor, it has been attempted to reduce the operating time to that fraction of the cycle in which knocking can occur. In fact, it has been found that during the running cycle of a cylinder, the knocking phenomenon only occurs during the beginning of the power stroke and, more precisely, within a range up to about 30.degree. of crank shaft rotation after the top dead centre, whereas the complete power stroke of a piston continues until 180.degree. after the top dead centre. It is seen, therefore, that it is desirable to be able to trigger the functioning of the knock sensor by means of a signal which appears at, or slightly after, the top dead centre position and which disappears about 30.degree. after the top dead centre. Clearly, this signal would be produced by means of a proximity sensor which is driven by the engine and produces a signal throughout the travel of the sensor through an angular range; it would also be possible to use a position sensor identifying the top dead centre position, and to define therefrom the beginning and the end of the desired sensor triggering signal by means of an electronic circuit. However, this procedure requires the use of a specific sensor for triggering the operation of the vibration sensor intended to prevent knocking. With a view to reducing cost, it is obviously desirable to reduce to a minimum the number of sensors to be employed, and the object of the invention is consequently to trigger the functioning of the antiknock vibration sensor at the opportune moment, without resorting to a proximity or position sensor identifying the top dead centre position, simply by using the ignition signal produced by the distributor.
The difficulty arises from the fact that this ignition signal is timed to occur at an instant which is shifted relative to the top dead centre position of the piston, the delay being a function of the rotation speed of the engine.